Question

An inductor L of inductance $X_L$ is connected in series with a bulb B and an ac source. How would brightness of the bulb change when (i) number of turn in the inductor is reduced. (ii) an iron rod is inserted in the inductor and (iii) a capacitor of reactance $X_C=X_L$ is inserted in series in the circuit. Justify your answer in each case

Answer 1

In the circuit,

$V_R+V_L=V$

$⟹V_R=V-V_L$

i) When the number of turns is reduced, its Inductance (L) reduces hence $V_L$. So $V_R$ increases thereby the brightness increases.

ii) When an iron rod is inserted, L increases, which implies $V_L$ increases means $V_R$

decreases. Thus brightness decreases.

iii) When a capacitor of reactance is inserted such that XC = XL ; Resonance condition is achieved;

which means that the whole voltage supply effectively drops across the resistance only. Thus, the brightness is greatest as VR is highest in this case.